Haili Zhaoa,
Peng Chen*a,
Yu Fana,
Junkai Zhang
b,
HongSheng Jiab,
Jianxun Zhaoa,
Heng Liua,
Xin Guo
a,
Xinwei Wanga and
Wanqiang Liu
*a
aSchool of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China. E-mail: chenp044@nenu.edu.cn; wqliu1979@126.com
bChina Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, China
First published on 6th January 2022
To improve the performance of lithium-sulfur (Li–S) batteries, herein, based on the idea of designing a material that can adsorb polysulfides and improve the reaction kinetics, a Co,N-co-doped graphene composite (Co–N–G) was prepared. According to the characterization of Co–N–G, there was a homogeneous and dispersed distribution of N and Co active sites embedded in the Co–N–G sample. The 2D sheet-like microstructure and Co, N with a strong binding energy provided significant physical and chemical adsorption functions, which are conducive to the bonding S and suppression of LiPSs. Moreover, the dispersed Co and N as catalysts promoted the reaction kinetics in Li–S batteries via the reutilization of LiPSs and reduced the electrochemical resistance. Thus, the discharge specific capacity in the first cycle for the Co–N–G/S battery reached 1255.7 mA h g−1 at 0.2C. After 100 cycles, it could still reach 803.0 mA h g−1, with a retention rate of about 64%. This phenomenon proves that this type of Co–N–G composite with Co and N catalysts plays an effective role in improving the performance of batteries and can be further studied in Li–S batteries.
Many researchers have focused substantial effort to solve the above-mentioned problems, adopting various strategies such as modifying the separator,22,23 exploiting new electrolytes,24,25 protecting the Li anode,26,27 and improving the sulfur hosting.8,28,29 One of the common methods is designing unique nanostructure materials to host sulfur by physical adsorption or/and chemical bonding.30 Graphene has special significance as a sulfur host material because of its atomically thin 2D structure, large specific surface area, good conductivity, catalytic activity, and excellent mechanical properties.31–33 It has been reported that the graphene can be combined with polar metal oxides, metal sulfides, and metal nitrides as active sites to anchor LiPSs by strong chemical bonding.34 The combination of graphene with transition metals effectively solves its problems, such as irreversible agglomeration, non-porous structure, and nonpolar physical barrier, and further enhances its electrochemical properties.35 However, transition metal/graphene compounds still exhibit the following key issues. Firstly, their interaction with the polar polysulfides is very weak. Secondly, their intrinsic low electronic conductivity increases the internal resistance. In addition, their high weight density counteracts the superior energy density of Li–S batteries.
Recently, much effort has been devoted to the development of nano-electrocatalysts with a high surface free energy, which not only can trap LiPSs due to their superior polarity, but also serve as electrocatalytic centers with sufficient exposure and accessibility.36 Moreover, transition metal catalysts on graphene ensure it acts as a conductive and flexible mechanical host for sulfur. Qiu et al. fabricated cobalt-embedded nitrogen-doped hollow carbon nanorods as sulfur hosts.9,37 Li et al. prepared N-doped carbon dodecahedron-supported Co as a sulfur host, exhibiting a good rate performance and recyclability.38 Kong and co-workers reported a cobalt-based catalyst for Li–S batteries with a high content of sulfur, certifying that the electrocatalysis enabled the accelerated formation and decomposition of lithium sulfides during the cycling processes.39 Huang et al. provided a porphyrin-derived atomic catalyst and discussed the dynamics of LiPSs.40 All these researchers have proven that transition metals can effectively promote the kinetics in Li–S batteries, resulting in an enhancement in their electrochemical performance. However, these methods usually involve sophisticated design and complicated processes, further hindering the scalable application of Li–S batteries. In this regard, it is urgent to design and develop simple and universal methods to produce transition metal/graphene composite-supporting electrocatalysts for high-performance Li–S batteries. Moreover, this method can open a new avenue for the doping of other metals in carbon materials.
In this work, we synthesized Co and N-doped graphene (Co–N–G) with well-dispersed Co and N catalysts embedded in graphene, which was used as a multi-functional anchor material for Li–S batteries. The Co, N catalysts were uniformly dispersed in graphene, hindering the crossover of polysulfides due to their strong physisorption–chemisorption. Moreover, the Co and N catalysts have a synergistic facilitation effect for the conversion of LiPSs into soluble short-chain LiPSs, reduction of short-chain LiPSs into Li2S and the oxidation of Li2S to sulfur, promoting the reaction kinetics during the battery charge and discharge process. Consequently, the battery with the Co–N–G cathode displayed the discharge specific capacity in the first cycle of 1255.7 mA h g−1 at 0.2C, concurrently with a satisfactory capacity retention of 64% after 100 cycles. This demonstrates the great potential of atom-scale Co–N–G composites for application in high-energy Li–S batteries.
We used a modified solvothermal method with AF as an intermediary to synthesize the monodispersed Co catalyst embedded in nitrogen-doped graphene. The morphology and microstructure of Co–N–G were investigated using SEM, TEM and HAADF-STEM. The SEM images of the pristine RGO and the Co–N–G composite are shown in Fig. 2a and b. The Co–N–G composite still maintained a complete layer structure and smooth surface and no large cobalt nanoparticles or big clusters were formed on the sample. In the TEM image in Fig. 2c, the wrinkles on the Co–N–G surface can be seen clearly, proving the well-defined 2D sheet-like microstructure of the Co–N–G composite. The 2D sheet structure is beneficial to significantly improve the sulfur loading and effectively enhance the physical adsorption to trap LiPSs owing to its large specific surface area.41 To directly investigate the morphology of the Co catalyst on graphene, we performed resolution aberration-corrected HAADF-STEM for the Co–N–G sample. The image in Fig. 2d shows numerous individual bright dots randomly dispersed in the 2D nanosheets, suggesting that the Co catalyst did not exhibit obvious serious agglomeration and was embedded on graphene.42 Moreover, in Fig. 2e, there are some red circles marking the aligned lattice fringes in the HRTEM image of Co–N–G, corresponding to the nano Co catalyst. The EDAX mapping images of Co–N–G are shown in Fig. 2f–i, which indicate that there is a homogeneous and dispersive distribution of C, N, and Co elements in the Co–N–G sample.43
The XRD spectra of Co–N–G and graphene were further analyzed to investigate their crystal structure, where only one diffraction peak at 26.4°, corresponding to the (002) crystal plane, was observed for both Co–N–G and graphene (Fig. 3a).44 This result indicates that no large cobalt-containing clusters could be detected by XRD examination, which is consistent with the result from the HRTEM images. Obviously, with the participation of Co and N, the intensity of the diffraction peak was reduced significantly. This implies that the sp2 ordered arrangement in the carbon catalyst became irregular with the embedding of Co and N.32 According to the analysis of Raman spectroscopy on Co–N–G, there are two characteristic peaks located at 1345 cm−1 for the D peak and 1575 cm−1 for the G peak (Fig. 3b).45 The value of ID/IG is usually used to indicate the disorder degree of the CC vibration mode. By calculation, the values of the ID/IG ratios of graphene and Co–N–G are 0.17 and 0.52, respectively.46 This proves that the atomic arrangement is more irregular for Co–N–G than graphene, suggesting the doping of Co and N. The sulfur content measured by TGA for the composite was 66.3 wt% (Fig. 3c). It was observed that the vaporization of sulfur started at around 150 °C and was completed at around 300 °C. The composition and the bonding between the catalyst and graphene were investigated by XPS measurement. As shown in Fig. 3d, the elements of C, N, O and Co can be detected in the XPS full spectrum of the Co–N–G sample. In the XPS spectra of C element (Fig. 3e), the peaks at 286.7, 285.5 and 284.8 eV correspond to the C–O, C–N and C–C/C
C bonds, indicating the interaction between the introduced heterocatalyst and the carbon matrix.47 The XPS Co2p spectrum in Fig. 3f shows that the Co species are present in the oxidation state of Co2+ at 781.8 eV, confirming the coordination configuration of Co bonding in Co–N.48,49 Three peaks for pyridinic, pyrrolic, and oxidized N groups can be observed in the fitting spectrum of N 1s for the samples of Co–N/G and N/G (Fig. 3g and h, respectively). The N 1s peak assigned to pyridinic N in Co–N–G shifted by 0.4 eV for N–G, indicating the Co catalyst bonded to pyridinic N.50 N doping gives rich Lewis basic sites, which are beneficial for chemical adsorption and the catalytic effect.51
The photograph and UV-vis spectra of the Li2S4 solutions with and without Co–N–G are shown in Fig. 4a. The absorption peak intensity of the Li2S4 solutions treated with Co–N–G was lower than that of the pristine solution, revealing that Co–N–G had superior Li2S4 adsorption ability. To detect the chemical adsorption ability of Co–N–G, XPS measurements on the Li2S4-absorbed Co–N–G samples were carried out. In Fig. 4b, there are three obvious binding energy peaks for Coma2p3/2 at 780.4 eV, Comi2p3/2 at 779.9 eV and Co–N at 782.2 eV in the Co 2p XPS patten. Compared to Co–N–G without Li2S4 (Fig. 3f), two overlapping peaks for Coma2p3/2 and Comi2p3/2 emerged, and the binding peak for Co–N shifted by about 0.4 eV. Both phenomena indicate the formation of the S–Co bond, confirming the stronger interaction of Li2S4–Co.52,53 In Fig. 4c, we found that the bonding peaks of N+–O−, Ngr and Npyr in the Li2S4-absorbed Co–N–G all show slight shifts, and Npyr exhibits a binding energy shift of 0.3 eV compared with that for the Li2S4-untreated sample, implying that the N element in Co–N–G also has adsorption power.38 The S2p spectrum shows binding energy peaks at 169.6, 168.4, 166.9, and 166.1 eV (Fig. 4d), which correspond to Co–S, S–O, S2p1/2 and S2p3/2 assigned to S–S and S–C, respectively.53,54 The results of the UV-vis absorption spectra and XPS spectra explained the Li2S4 adsorption capability of Co–N–G.
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Fig. 4 (a) UV-vis absorption spectra and photograph (inset) of Li2S4 solutions with and without Co–N–G. (b) Co 2p, (c) N 1s and (d) S 2p XPS spectra of the Li2S4-absorbed samples. |
The overall electrochemical performances are usually used to demonstrate the catalytic effect for the conversion of lithium polysulfides by analyzing their conversion reaction rate and the lithium-ion diffusivity. The electrochemical measurements were performed using a sulfur areal loading of 3.6 mg cm−2 at a potential of 1.7 to 2.8 V (vs. Li/Li+).55 As the reference sample, the cell with a pristine graphene electrode was fabricated under the same conditions. The Nyquist plot and the phase-Bode plot were measured after 50 cycles, as shown in Fig. 5a and b, respectively. The ohmic resistance (Rs) comes from the electrolyte resistance. This semi-circle at HF (100 kHz–1 kHz) is associated with the interfacial resistance (Rint) of the Li surface or the contact points between carbon and sulfur. This semi-circle at MF (1 kHz to 1 Hz) is related to the polysulfide is formed in the electrolyte, indicating the charge transfer reaction of the actual reduction/oxidation reaction of polysulfide. The (Rct) charge-transfer resistance reflects the charge-transfer process at the interface between the conductive agent and the electrolyte. The fitting values are presented in Table 1. The Co–N–G electrode presented a lower Rct than that of the G/S electrode. In an Li–S battery, a lower Rct is often related to less cumulative agglomerates, indicating an increase in electrical conductivity on the cathode surface, enhancement ion transportation, and a rapid ionic exchange process.56 Thus, the EIS results suggest that the Co and N catalysts effectively improved the reaction kinetics in the Li–S battery.
Sample | Rs/Ω | Rint/Ω | Rct/Ω |
---|---|---|---|
G/S | 6.024 | 8.397 | 13.27 |
Co–N–G/S | 3.888 | 3.708 | 6.226 |
The oxidation–reduction reactions on the electrodes were explored via CV tests, as shown in Fig. 5c. For the Co–N–G cell, there are two reduction peaks and one oxidation peak, corresponding to the conversion of sulfur to LiPSs, reduction of short-chain LiPSs to Li2S and the oxidation of Li2S to sulfur, respectively. By comparison, a narrower potential hysteresis and higher peak currents and area were observed for the Co–N–G electrode than that in the pristine graphene electrode, representing the rapid catalytic ability of the doped Co–N. The potential difference ΔECo–N–G between the oxidation and reduction platform of the Co–N–G/S cell is 0.34 V, and the electrode potential difference ΔEG for the pristine cell is 0.51 V. The polarization of the Co–N–G battery is relatively weaker, proving that the introduction of the Co and N catalysts relieved the electrochemical energy barrier due to their superior catalytic effect.57 Besides, the charge–discharge platform curves are exhibited in Fig. 5d, which are consistent with the CV results. To further explore the reaction kinetics, the transport rate of lithium ions was analyzed by CV scanning at different rates, which was carried out in the low-frequency region in the impedance map.58 With an increase in the CV scan rate from 0.1 to 0.5 mV s−1 (Fig. 5e), the redox-reduction peak exhibited a slight shift, confirming the catalytic activity of the Co atom. To calculate the diffusion coefficient of lithium ions, the fitted Z′ and ω−0.5 of the electrodes after 50 cycles were calculated, where Z′ is the actual measured Warburg impedance and ω represents the angular frequency, following a linear function relationship, as shown in eqn (1). The specific value of the Weber factor σ was obtained by calculating the slope in Fig. 5f. The lithium-ion diffusion coefficient DLi+ can be obtained using eqn (2), the absolute temperature T, the gas constant R, the Faraday constant F, the normalized area of the positive electrode A, the lithium-ion concentration CLi+, and the number of electrons participating in the reaction.59 The lithium-ion diffusion coefficients are presented in Table 2, indicating that the Co–N–G electrode has fast ion transport. Thus, the results of the electrochemical measurements prove that the Co–N–G/S cell has lower resistances, lower electrode potential difference, ΔE, and faster ion transport than that of the G/S cell, resulting in a rapid electronic and ionic exchange process in the Co–N–G/S cell. Thus, it can be concluded that the Co–N–G electrode exhibits significant catalytic effect to enhance the reaction kinetics in Li–S batteries.
Z′ = Rs + Rct + σω−0.5 | (1) |
![]() | (2) |
G/S | Co–N–G/S | |
---|---|---|
σ | 7.15 | 5.17 |
DLi+/10−17 (cm2 s−1) | 3.83 | 7.32 |
Base on the superior function of Co–N–G in catalytic action for improving the kinetics of the conversion reaction, the Li–S battery exhibited an excellent performance. The Co–N–G electrode battery delivered a high capacity of 1255.7 mA h g−1 with almost 64% retention of its initial capacity at 0.2C after 100 cycles (Fig. 6a). The good rate performance is shown in Fig. 6b, where even at 2C, the Co–N–G electrode endowed the batteries with a high reversible capacity of 704.6 mA h g−1, which is almost twice that in the pristine batteries. This further confirms the superior catalytic activity of the active Co and N catalysts in trapping the immobilized LiPSs and the enhanced kinetic conversion of the LiPSs on Co–N–G. To further explore the Co–N–G battery, its long cycling performance at a larger current density of 1C and 2C was investigated (Fig. 6c). Interestingly, the battery with Co–N–G had the highest initial capacity of 821.9 mA h g−1 at 1C and 736.0 mA h g−1 at 2C, with about 60% and 45% capacity retention after 500 cycles, respectively. The enhanced performance is ascribed to the accelerated conversion reaction of polysulfide intermediates, resulting from the significant adsorption ability and catalysis of Co–N–C.
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